Advances in Mathematical Physics

Volume 2015 (2015), Article ID 683658, 5 pages

http://dx.doi.org/10.1155/2015/683658

## Entropic Lower Bound for Distinguishability of Quantum States

^{1}Center for Macroscopic Quantum Control, Department of Physics and Astronomy, Seoul National University, Seoul 151-742, Republic of Korea^{2}Department of Physics, Hanyang University, Seoul 133-791, Republic of Korea

Received 18 August 2015; Accepted 22 October 2015

Academic Editor: Remi Léandre

Copyright © 2015 Seungho Yang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

For a system randomly prepared in a number of quantum states, we present a lower bound for the distinguishability of the quantum states, that is, the success probability of determining the states in the form of entropy. When the states are all pure, acquiring the entropic lower bound requires only the density operator and the number of the possible states. This entropic bound shows a relation between the von Neumann entropy and the distinguishability.

#### 1. Introduction

Quantum mechanics does not allow determining the state of a system by measuring a single copy of the ensemble. Nevertheless, if some prior information is known, it is possible to guess the state with a certain degree of confidence even by a single measurement. Given some prior information, to what extent quantum states can be distinguished is an intriguing issue from both fundamental and practical points of view. For example, this problem is closely related to efficiencies of quantum communication [1–3]. It is known that the imperfect distinguishability plays a crucial role in the security of quantum cryptography [3].

There are different approaches to the distinguishability of quantum states [4–7]. In the minimum-error discrimination problem [4], a set of known quantum states and preparation probabilities are given, and one aims to distinguish the states with the optimal probability of success. The optimal success probability is an operationally well-defined measure for the distinguishability of given states. However, it is a highly demanding quest to find its analytical solution for general sets of states, and the solution is only known for the sets of two states [1]. Instead, upper bounds [8–12] and lower bounds [13–15] have been provided to estimate the optimal probability. On the other hand, there have been studies of distinguishability between unknown quantum states using programmable machines [6, 7]. In this case, ancillary systems prepared in each of the unknown states are provided as an input of the machine.

One may pay attention to the von Neumann entropy as a quantity related to the distinguishability in light of the capacity of a quantum state for embodying quantum information. When a system is probabilistically prepared in one of a certain number of quantum states, its state of the statistical mixture is described by a density operator. According to the quantum source theorem [16], the von Neumann entropy of the system, which is given as a function of the density operator, represents the capacity of the mixed state to (asymptotically) carry quantum information. As discussed in [17], one may relate this capacity to the concept of distinguishability because more information could be carried when each state is more distinguishable. For this reason, Jozsa and Schlienz considered the von Neumann entropy as measure for distinguishability [17]. However, it is not known whether this kind of distinguishability is linked to the actual ability to distinguish quantum states by measurements, namely, the success probability (distinguishability will henceforth refer to the success probability). It seems that, at least, the von Neumann entropy cannot pinpoint the success probability of distinguishing given quantum states. This is because the von Neumann entropy is determined only by the density operator, and the density operator of a system can take arbitrarily many decompositions in general; thus it does not contain information on which states the system could have been prepared in.

We here present a lower bound for the distinguishability, that is, the optimal success probabilities of distinguishing between quantum states, as a function of entropies of the system. For a system prepared in one of pure states, we also present a reduced form of the entropic bound which requires the density operator and the number of possible states for its evaluation. It reveals a relation between the von Neumann entropy and the distinguishability; the larger von Neumann entropy guarantees the better distinguishability.

#### 2. General Formulation

Consider a quantum system prepared in one of quantum states with some probabilities. We denote them by . One wishes to identify which state the system has been prepared in or, equivalently, to identify the value of . The value of is determined, using a generalized measurement described by the measurement operator . Therefore, the probability of correctly identifying for a given is , and the expected success probability isWhen maximized over all measurements, it becomes the optimal success probability, which is denoted by . It is the quantity which we consider as the degree of the distinguishability of quantum states. As already mentioned, however, the analytical form of is known only for the two-state case [1].

An equivalent way of describing the scenario is to consider a classical-quantum system in the state,where the indices ’s are encoded. Namely, one is given the quantum system and wishes to determine the value of by measuring . In terms of entropic quantities, has uncertainty quantified by Shannon entropy , and it has a correlation with the quantum system.

One may expect an entropic lower bound from the intuition that the correlation of with would enhance the distinguishability of the quantum states. For better understanding, let us first consider a fully classical case [18], where the quantum system is replaced with a classical system and . Assume that we are given and wish to determine the value of from . For given , the most probable is the one that gives the maximum conditional probability, . Therefore, the optimal success probability is attained by choosing them for all , and it is given as . It can be lower bounded in terms of the correlation between and as follows: The first inequality follows by taking the average of over , and the second one follows from the concavity of the exponential function. The exponent in the last line is equal to the conditional Shannon entorpy so thatwhere is the classical mutual information between and . Therefore, with assistance from the random variable having the amount of correlation , one can guess the random variable with probability (in logarithm) at least .

#### 3. Entropic Lower Bound for the Distinguishability of Quantum States

For the quantum case, we can still obtain a random variable by applying a measurement (i.e., the outcome of the measurement can be considered as random variable), and applying (4) gives . However, this bound can be further sharpened by using the quantum entropies. For a quantum system prepared in a density operator , the von Neumann entropy is defined as . The conditional von Neumann entropy of a bipartite system is defined as . Similarly, defines the von Neumann mutual information [19]. In terms of the quantum entropies, we present the quantum entropic lower bound.

Theorem 1. *For a set of quantum states with preparation probabilities , the optimal success probability of distinguishing the quantum states, , is lower bounded as*

*Proof. *For the proof, we employ the conditional min-entropy [20], which has many applications in quantum cryptography. The conditional min-entropy of a system in a state is defined as where denotes the set of all quantum states of the subsystem . We derive the lower bound (5) from two properties of the conditional min-entropy. First, the conditional min-entropy has an operational meaning that, for the classical-quantum states in (2), the logarithm of is equal to the negative conditional min-entropy (see [21] for the details). Therefore, for the classical-quantum state in (2),It has also been shown in [22] that the conditional min-entropy is always less than or equal to the conditional von Neumann entropy, so we haveUsing (7) and (8), we obtainwhich is equivalent to the first line in (5). On the other hand, the conditional von Neumann entropy satisfies the chain rule, . It enables the lower bound to be written as a function of entropies of the system . The von Neumann entropies of and are evaluated as and . It then follows that , and this gives the second line in (5).

Let us take a closer look into the form of the lower bound. Its form is exactly of (4), but only is replaced with . The quantum mutual information is known to capture all the correlations including both classical and quantum parts [23, 24], so it is considered as a measure of the total correlations. Hence, we see that the lower bound is increased by the total correlation. For the classical-quantum state , we obtain , and Holevo’s theorem [25, 26] implies that for any measurement on the system ( is the outcome of a measurement on ). The minimum difference between and , that is, , is equal to quantum discord [27] of .

On the assumption that the system is prepared in one of pure states , the entropic bound can be reduced to a form that only requires the density operator of the system and the number of the possible states for its evaluation. Using and , we have

Therefore, we see that the larger von Neumann entropy guarantees the better distinguishability. Notwithstanding the missing information on the component states and the preparation probabilities, the density operator of a system alone can provide a lower bound for distinguishability of pure states. Note that when many copies of quantum systems are prepared in the same way, the density operator can be obtained using the state tomography, but it is impossible to guess the component states. The state-discrimination machine with unknown quantum states as an input [6, 7] is the case where it is required to distinguish between unknown quantum states.

#### 4. Other Lower Bounds and Examples

In this section, we compare the entropic lower bound to other previously known bounds. One is the lower bound given by the square-root measurement [13], and it is known to be optimal for many cases in which the solutions are known [4]. The measurement operators of the square-root measurement are given as where . Therefore, a lower bound by the square-root measurement is given as

Another one is the pairwise-overlap bound. For an ensemble of pure states , it has been given in [14] asThis was derived as a lower bound for the square-root measurement bound, so it is always less than or equal to the bound by the square-root measurement. It provides an analytic form of lower bounds in terms of the pairwise overlaps.

We now consider a few exemplary sets of pure states and compare the entropic bound in (10) with the other two lower bounds. Let us first look at three 3-dimensional pure states with equal probabilities:The entropic lower bound and the other two bounds are calculated and plotted in Figure 1(a). In Figure 1(b), we present the bound values with another set of states, where only is replaced with from the previous case. As shown in Figure 1, for both the sets of states, the square-root measurement provides the tightest bounds. The entropic bound is shown to be greater than the pairwise-overlap bound for large regions.